This invention relates to electric vehicles and more particularly to a battery reconnect for use on an electric vehicle driven by a separately excited field controlled direct current drive motor.
In most prior art electric vehicle speed control systems, a direct current (DC) chopper control and a series wound drive motor are used. The chopper is placed in series between a DC power source and the series wound drive motor. The DC chopper controls the speed of the DC series motor by controlling and interrupting the full armature current. The speed of the DC drive motor is controlled by a conventional armature chopper controller which provides complete control of the motor over its entire speed range. The prior art DC chopper controls the time of the pulses and/or the magnitude of the pulses being fed through the armature of the DC drive motor. By varying the parameters of the driving pulses being fed to the armature of the DC drive motor, the speed of the drive motor can be varied. DC chopper control for armature current coupled with a series wound motor is the most commonly used approach for driving prior art electric vehicles. Since the chopper must control the full armature current, it must of necessity be of a large size to handle the large currents involved. Prior art DC choppers required in the armature circuit are bulky and expensive. The conventional chopper controller since it must interrupt and control full armature current requires expensive high powered thyristors and complicated control circuitry for switching and controlling the load current in the armature circuit. Due to the large currents being interrupted cooling of the controller is a problem. Conventional chopper controllers for armature current are also rather inefficient at low to normal operating speeds.
An electronic system for controlling the torque speed characteristics and regenerative braking of a separately excited drive motor for use on electric vehicles is disclosed in cross referenced U.S. Pat. application Ser. No. 346,552. The torque speed characteristics can be controlled by varying the separately excited shunt field of the DC drive motor. As disclosed in the above-mentioned application, the speed of the DC drive motor is controlled from some base speed to a maximum speed. A problem with the speed control as there taught is that the base speed of the DC drive motor is fixed at a relatively high value. It is desirable to be able to operate the DC drive motor at a speed lower than the normal operating base speed. This can be accomplished by connecting the batteries in parallel so as to obtain lower output voltage. The output voltage determines the base speed of the DC drive motor. By providing for series and parallel operation of the batteries two base speeds can be achieved. In prior art connecting schemes, as the switches are opened disconnecting the batteries from one configuration, switches are closed connecting the batteries in another configuration. That is, the batteries are almost simultaneously switched from a connection yielding one voltage level to another connection giving a different voltage level. When this direct switching is attempted while connected to a direct current drive motor, having a separately excited field, excessive current transients and vehicle jerk result. It is desirable to have a battery reconnect means which can change the supply voltage level without excessive current transients or jerky vehicle operation. It is also desirable to have an electric vehicle in which the batteries can be connected in the high voltage configuration at the lowest practical speed. This is desirable since operating efficiency and performance are improved while in the high voltage connection. It is also desirable to have an electric vehicle which provides for rapid acceleration when the throttle is substantially depressed.
At low vehicle speed it is only possible to operate at the lower battery voltage, which is normally obtained with a parallel battery connection, because the obtainable back emf of the motor is inadequate to maintain control of the armature current for the higher voltage level, obtained with the series battery connection. Once a high enough speed is reached, operation from either high voltage or low voltage is possible. Since the performance and efficiency are considerably better for the high voltage connection, it is important to select the high voltage connection whenever possible.